Attempts to increase the capacity of magnetic data storage devices must balance writability, grain size and magnetic anisotropy in the magnetic data storage media. Write heads can only generate a limited magnetic field, and this limit is set by the maximum volume magnetization that can be achieved in a material, the maximum current density that can be put through a conductor, and the head-to-media separation. If the anisotropy in the media is lowered to the point where it can be written by the write head and the grains are made small enough to maintain an acceptable signal-to-noise ratio, the media may not be thermally stable for large areal densities. This is referred to as the superparamagnetic limit.
Ferroelectric (FE) data storage media has the advantage that it is written using an electric field, and very large electric field values can be generated with a thin-film device. Thus, FE media with a very large anisotropy can be written by a thin-film device, and a thermally stable FE media with very small domains (and narrow domain walls) can be written. One of the problems with using FE recording is that the readback is difficult. Free electric charge tends to shield the polarization pattern in the FE media, which then requires one to use a write before read method for readback. Since free magnetic charges don't exist, this is not a problem for magnetic media and readback is relatively easy.
Recently, composite materials, combining magnetoelastic and ferroelectric materials, have been developed that have both ferromagnetic (FM) and FE properties above room temperature. Two example composites are BiFeO3—CoFe2O4 and BaTiO3—CoFe2O4. In these examples the BiFeO3 and BaTiO3 are the FE materials and the CoFe2O4 is the FM material.
Others have demonstrated the use of an electric field to switch the magnetization in a BiFeO3—CoFe2O4 composite. The composite consisted of CoFe2O4 nanopillars in a BiFeO3 matrix. The material was deposited using pulsed laser deposition at 700° C. on SrRuO3 and resulted in a fully epitaxial film, including across the BiFeO3—CoFe2O4 grain boundary. The magnetization was saturated by applying a large, uniform magnetic field, and then 50 to 60% of the magnetization was switched by applying an electric field using a piezoelectric force microscope (PFM). The films referred to above were not used for data storage.
There remains a need for a magnetic data storage method and apparatus that can provide adequate thermal stability and adequate signal-to-noise ratio (SNR).